The present invention relates to a piston pump, which is appropriate in particular for the pressure fluid delivery in electronically controlled brake systems, including a pump piston that is designed as a stepped piston.
A generic stepped piston pump is e.g. disclosed in German patent application DE 43 16 986 A1. Pumps with a stepped piston of this type are employed especially in electronically controlled brake systems where they cause a reduction of the aspiration resistance of the pump by means of a charging effect. In this arrangement, the pressure in a supply chamber on the suction side of the pump will be increased due to the stepped piston during a suction stroke of the stepped piston pump. An improved delivery rate of the pump and a quicker response time of the electronically controlled brake system is achieved thereby in particular at low temperatures and low viscosity of the pressure fluid. This is especially important in the case of traction slip control or driving stability control operations.
However, especially in these applications the generic piston pump suffers from the disadvantage that the pump piston is guided directly in the housing. Chips may develop due to wear between the piston and the housing and may get into the brake circuit. Therefore, the bearing surface must undergo a special machining operation which causes additional costs of manufacture. Further, this piston pump necessitates a relatively large mounting space due to the axial series connection of the pressure valve and the suction valve.
An object of the present invention is to reduce the manufacturing costs for a generic piston pump and to improve the reliability in operation of the pump. Another objective is to minimize the number of assemblies.
This object is achieved according to the present invention in that the stepped piston of a generic piston pump at its large-diameter portion is guided in a pump cartridge. The inner side of the cartridge serves as a bearing surface for the portion of the stepped piston that has the large diameter. Thus, this bearing surface advantageously permits either being machined outside the pump housing or being made with the corresponding required surface quality directly when the pump cartridge is manufactured.
It is preferred that the pressure valve is designed radially to the direction of movement of the stepped piston. This arrangement of the pressure valve renders it possible to reduce the mounting space that is required in an axial direction. It is especially favorable that the pump cartridge thus combines the functions of guiding the piston and retaining the pressure valve. According to the present invention, this reduces the number of the necessary assemblies which leads to another cost reduction. For ease of manufacture or machining the pump cartridge is preferably made of plastics or a sintered metal.
In a favorable aspect of the present invention, a part of a suction channel of the piston pump is arranged close to the outside edge of the pump housing. This is advantageous because the suction channel provides a better access for connecting elements such as hydraulic lines, etc., at the outside end of the housing, which e.g. may be the valve block of a brake assembly. Preferably, another part of the suction channel is provided in the pump cartridge. The suction path of the pressure fluid from the connecting element to the suction valve hence leads through the cartridge in particular. Preferably, another part of the suction channel extends through the stepped piston itself, and the suction valve is designed directly or indirectly on the stepped piston.
It is particularly favorable that a filter element is provided in the suction channel between the pump cartridge and the stepped piston. According to the present invention, the filter element protects at this preferred location both the suction valve and the subsequent pressure valve against contaminants.
To seal the supply chamber against the drive elements of the piston pump, a seal is further provided advantageously in the pump housing adjoining the small-diameter portion of the stepped piston. This seal is not arranged at the piston but in a bore in the housing, which is in contrast to the state of the art. In addition or separately, a wear-resistant element may be provided in the pump housing and serve in a favorable way as a bearing surface for that part of the stepped piston which has a small diameter.
It is especially advantageous that the seal and/or the wear-resistant element is retained in the housing by means of the filter element, whereby an additional retaining ring becomes unnecessary and the number of component parts is further reduced.
The housing of the piston pump of the present invention is closed towards the outside in particular by means of a cover which is calked or clinched in the housing. The cover may be designed integrally with the pump cartridge, or separately.